matplotlib · tacaswell · Aug 28, 2018 · Aug 28, 2018
diff --git a/lib/matplotlib/__init__.py b/lib/matplotlib/__init__.py
@@ -138,7 +138,7 @@
 
 # cbook must import matplotlib only within function
 # definitions, so it is safe to import from it here.
-from . import cbook
+from . import cbook, rcsetup
 from matplotlib.cbook import (
     MatplotlibDeprecationWarning, dedent, get_label, sanitize_sequence)
 from matplotlib.cbook import mplDeprecation  # deprecated
@@ -853,6 +853,10 @@ def __setitem__(self, key, val):
                 cbook.warn_deprecated(
                     "3.0", "{} is deprecated; in the future, examples will be "
                     "found relative to the 'datapath' directory.".format(key))
+            elif key == 'backend':
+                if val is rcsetup._auto_backend_sentinel:
+                    if 'backend' in self:
+                        return
             try:
                 cval = self.validate[key](val)
             except ValueError as ve:
@@ -881,6 +885,12 @@ def __getitem__(self, key):
                 "3.0", "{} is deprecated; in the future, examples will be "
                 "found relative to the 'datapath' directory.".format(key))
 
+        elif key == "backend":
+            val = dict.__getitem__(self, key)
+            if val is rcsetup._auto_backend_sentinel:
+                from matplotlib import pyplot as plt
+                plt.switch_backend(rcsetup._auto_backend_sentinel)
+
         return dict.__getitem__(self, key)
 
     def __repr__(self):
@@ -1095,10 +1105,10 @@ def rc_params_from_file(fname, fail_on_error=False, use_default_template=True):
         _fullpath = os.path.join(_basedir, rcParams['examples.directory'])
         rcParams['examples.directory'] = _fullpath
 
-rcParamsOrig = rcParams.copy()
 
 with warnings.catch_warnings():
     warnings.simplefilter("ignore", MatplotlibDeprecationWarning)
+    rcParamsOrig = RcParams(rcParams.copy())
     rcParamsDefault = RcParams([(key, default) for key, (default, converter) in
                                 defaultParams.items()
                                 if key not in _all_deprecated])
@@ -1222,7 +1232,7 @@ def rc_file_defaults():
     with warnings.catch_warnings():
         warnings.simplefilter("ignore", mplDeprecation)
         from .style.core import STYLE_BLACKLIST
-        rcParams.update({k: v for k, v in rcParamsOrig.items()
+        rcParams.update({k: rcParamsOrig[k] for k in rcParamsOrig
                          if k not in STYLE_BLACKLIST})
 
 
@@ -1238,7 +1248,8 @@ def rc_file(fname):
     with warnings.catch_warnings():
         warnings.simplefilter("ignore", mplDeprecation)
         from .style.core import STYLE_BLACKLIST
-        rcParams.update({k: v for k, v in rc_params_from_file(fname).items()
+        rc_from_file = rc_params_from_file(fname)
+        rcParams.update({k: rc_from_file[k] for k in rc_from_file
                          if k not in STYLE_BLACKLIST})
 
 
@@ -1289,16 +1300,23 @@ def __init__(self, rc=None, fname=None):
             if rc:
                 rcParams.update(rc)
         except Exception:
-            # If anything goes wrong, revert to the original rcs.
-            dict.update(rcParams, self._orig)
+            self.__fallback()
             raise
 
+    def __fallback(self):
+        # If anything goes wrong, revert to the original rcs.
+        updated_backend = self._orig['backend']
+        dict.update(rcParams, self._orig)
+        # except for the backend.  If the context block triggered resloving
+        # the auto backend resolution keep that value around
+        if self._orig['backend'] is rcsetup._auto_backend_sentinel:
+            rcParams['backend'] = updated_backend
+
     def __enter__(self):
         return self
 
     def __exit__(self, exc_type, exc_value, exc_tb):
-        # No need to revalidate the original values.
-        dict.update(rcParams, self._orig)
+        self.__fallback()
 
 
 def use(arg, warn=True, force=False):
@@ -1324,14 +1342,14 @@ def use(arg, warn=True, force=False):
 
     force : bool, optional
         If True, attempt to switch the backend.  This defaults to
-        false and using `.pyplot.switch_backend` is preferred.
+        False.
 
 
     """
     name = validate_backend(arg)
 
     # if setting back to the same thing, do nothing
-    if (rcParams['backend'] == name):
+    if (dict.__getitem__(rcParams, 'backend') == name):
         pass
 
     # Check if we have already imported pyplot and triggered
@@ -1361,7 +1379,7 @@ def use(arg, warn=True, force=False):
 
 
 if os.environ.get('MPLBACKEND'):
-    use(os.environ['MPLBACKEND'])
+    rcParams['backend'] = os.environ.get('MPLBACKEND')
 
 
 def get_backend():

diff --git a/lib/matplotlib/backends/__init__.py b/lib/matplotlib/backends/__init__.py
@@ -10,12 +10,9 @@
 
 _log = logging.getLogger(__name__)
 
-backend = matplotlib.get_backend()
-# FIXME: Remove.
-_backend_loading_tb = "".join(
-    line for line in traceback.format_stack()
-    # Filter out line noise from importlib line.
-    if not line.startswith('  File "<frozen importlib._bootstrap'))
+
+# NOTE: plt.switch_backend() (called at import time) will add a "backend"
+# attribute here for backcompat.
 
 
 def _get_running_interactive_framework():

diff --git a/lib/matplotlib/pyplot.py b/lib/matplotlib/pyplot.py
@@ -39,7 +39,7 @@
 from matplotlib.backend_bases import FigureCanvasBase
 from matplotlib.figure import Figure, figaspect
 from matplotlib.gridspec import GridSpec
-from matplotlib import rcParams, rcParamsDefault, get_backend
+from matplotlib import rcParams, rcParamsDefault, get_backend, rcParamsOrig
 from matplotlib import rc_context
 from matplotlib.rcsetup import interactive_bk as _interactive_bk
 from matplotlib.artist import getp, get, Artist
@@ -67,7 +67,7 @@
            Locator, IndexLocator, FixedLocator, NullLocator,\
            LinearLocator, LogLocator, AutoLocator, MultipleLocator,\
            MaxNLocator
-from matplotlib.backends import pylab_setup
+from matplotlib.backends import pylab_setup, _get_running_interactive_framework
 
 _log = logging.getLogger(__name__)
 
@@ -78,35 +78,15 @@
 # FIXME: Deprecate.
 def _backend_selection():
     """
-    If rcParams['backend_fallback'] is true, check to see if the
-    current backend is compatible with the current running event loop,
-    and if not switches to a compatible one.
-    """
-    backend = rcParams['backend']
-    if not rcParams['backend_fallback'] or backend not in _interactive_bk:
-        return
-    is_agg_backend = rcParams['backend'].endswith('Agg')
-    if 'wx' in sys.modules and backend not in ('WX', 'WXAgg'):
-        import wx
-        if wx.App.IsMainLoopRunning():
-            rcParams['backend'] = 'wx' + 'Agg' * is_agg_backend
-    elif 'PyQt4.QtCore' in sys.modules and not backend == 'Qt4Agg':
-        import PyQt4.QtGui
-        if not PyQt4.QtGui.qApp.startingUp():
-            # The mainloop is running.
-            rcParams['backend'] = 'qt4Agg'
-    elif 'PyQt5.QtCore' in sys.modules and not backend == 'Qt5Agg':
-        import PyQt5.QtWidgets
-        if not PyQt5.QtWidgets.qApp.startingUp():
-            # The mainloop is running.
-            rcParams['backend'] = 'qt5Agg'
-    elif 'gtk' in sys.modules and 'gi' in sys.modules:
-        from gi.repository import GLib
-        if GLib.MainLoop().is_running():
-            rcParams['backend'] = 'GTK3Agg'
-    elif 'Tkinter' in sys.modules and not backend == 'TkAgg':
-        # import Tkinter
-        pass  # what if anything do we need to do for tkinter?
+    If rcParams['backend_fallback'] is true, we will check (at backend
+    load-time) to see if the current backend is compatible with the current
+    running event loop, and if not switches to a compatible one.
+    """
+    if rcParams["backend_fallback"]:
+        if (dict.__getitem__(rcParams, "backend") in _interactive_bk
+                and _get_running_interactive_framework()):
+            dict.__setitem__(
+                rcParams, "backend", rcsetup._auto_backend_sentinel)
 
 
 _backend_selection()
@@ -237,6 +217,7 @@ def switch_backend(newbackend):
             except ImportError:
                 continue
             else:
+                rcParamsOrig['backend'] = candidate
                 return
 
     backend_name = (

diff --git a/lib/matplotlib/testing/__init__.py b/lib/matplotlib/testing/__init__.py
@@ -36,7 +36,7 @@ def setup():
                 "Could not set locale to English/United States. "
                 "Some date-related tests may fail.")
 
-    mpl.use('Agg', warn=False)  # use Agg backend for these tests
+    mpl.use('Agg', force=True, warn=False)  # use Agg backend for these tests
 
     with warnings.catch_warnings():
         warnings.simplefilter("ignore", MatplotlibDeprecationWarning)

diff --git a/lib/matplotlib/testing/conftest.py b/lib/matplotlib/testing/conftest.py
@@ -8,7 +8,7 @@
 
 
 def pytest_configure(config):
-    matplotlib.use('agg')
+    matplotlib.use('agg', force=True)
     matplotlib._called_from_pytest = True
     matplotlib._init_tests()
 
@@ -53,6 +53,8 @@ def mpl_test_settings(request):
             if backend is not None:
                 plt.switch_backend(prev_backend)
 
+    assert matplotlib.get_backend() == 'agg'
+
 
 @pytest.fixture
 def mpl_image_comparison_parameters(request, extension):

diff --git a/lib/matplotlib/tests/test_backend_svg.py b/lib/matplotlib/tests/test_backend_svg.py
@@ -139,7 +139,7 @@ def test_determinism(filename, usetex):
                 [sys.executable, '-R', '-c',
                  'import matplotlib; '
                  'matplotlib._called_from_pytest = True; '
-                 'matplotlib.use("svg"); '
+                 'matplotlib.use("svg", force=True); '
                  'from matplotlib.tests.test_backend_svg '
                  'import _test_determinism_save;'
                  '_test_determinism_save(%r, %r)' % (filename, usetex)],